Injection and injection compression molding of ultra‐high‐molecular weight polyethylene powder

Abstract

Ultra‐high‐molecular weight polyethylene (UHMWPE) powder was processed using injection molding (IM) with different cavity thicknesses and injection‐compression molding (ICM). The processing parameters of feeding the powders were optimized to ensure proper dosage and avoid jeopardizing the UHMWPE molecular structure. Dynamic mechanical analysis (DMA) and Fourier‐transform infrared spectroscopy tests confirmed that the thermal and oxidative degradations of the material were avoided but crosslinking was induced during melt processing. Tensile tests and impact tests showed that the ICM samples were superior to those of IM. Increased cavity thickness and ICM were helpful for reducing the injection pressure and improving the mechanical properties due to effective packing of the material. Short shot molding showed that the UHMWPE melt did not exhibit the typical progressive and smooth melt front advancements. Due to its highly entangled polymer chains structure, it entered the cavity as an irregular porous‐like structure, as shown by short shots and micro‐computed tomography scans. A delamination skin layer (around 300‐μm thick and independent of cavity thickness) was formed on all IM sample surfaces while it was absent in the ICM samples, suggesting two different flow behaviors between IM and ICM during the packing phase. POLYM. ENG. SCI., 59:E170–E179, 2019. © 2018 Society of Plastics Engineers

Injection molding of delamination‐free ultra‐high‐molecular‐weight polyethylene

“…It is known that UHMWPE tends to exhibit strain hardening behavior unless the structure contains defects resulting from poor melt processing and/or a delamination layer . Therefore, the improvement in ultimate strength in Fig.…”

“…The calculated molecular weight was 5.0 million g/mol and the true density was 0.93 g/cm 3 . More details about the material can be found in our previous study . The melting point of UHMWPE is around 135°C, although it changes a lot depending on the degree of crystallinity.…”

“…The processing parameters are provided in Table 3. These parameters were used in injection-molding experiments for all samples and were selected according to our previous study [10]. The temperature of the hopper throat was adjusted lower than the room temperature at 18 C to have continuous and reliable powder feeding through the hopper.…”

“…Separate entangled chain bundles, especially those across the highly sheared layer near the frozen part surface, require a longer time and higher temperature to fuse to the core during the packing cycle . Increasing the packing pressure alone does not eliminate the delamination layer, rather it makes the delamination problem worse .…”

“…In another study, we reported that a powder UHMWPE resin was processed with regular injection molding, as well as a special process called injection–compression molding (ICM). For the regular injection molding of UHMWPE powder, the so‐called delamination layer problem was observed . This persistent layering problem occurred near the surface of the part by delamination of the skin from the core.…”